Cells IV Meiosis

What is Meiosis?  A special type of cell division used only for the production of gametes  Gametes are the reproductive cells of an organism  Typical mitosis cell division produces genetically identical daughter cells  Why wouldn’t this work for gametes?

Gamete Production  Human somatic cells (those not used for reproduction) have 46 chromosomes  A chromatid has a complete set of an organisms DNA  A chromosome has 2 copies of that set  This is called a “diploid number” of chromosomes (2x)  If gametes were made like somatic cells, through mitosis, the sperm and the egg would each have ______ chromosomes  This means that upon conception, a zygote would have ______ combined chromosomes from its parents

46 Fertilized Egg ?

Gamete Production  As a result of the problem of excess chromosomes, gametes are produced in a different way  Meiosis  Often called “reduction division”  ________________________ must be reduced, producing haploid cells  Haploid = ½ x the number of necessary chromosomes  _____________ must be divided

Chromosomes Segments of DNA found in our chromosomes contain genes Each chromatid has a complete set of our genetic code, the chromosome, then, has 2 copies of that complete code Since each of 2 parents donated chromosomes to us, they are said to come in pairs (23 pairs in humans). These pairs are said to be homologous pairs because they each have genes for the same traits. For example, a homologous pair might have the gene for eye color from the mother on one chromosome and a different gene for eye color from the father on the other.

The Phases  Meiosis begins with the same type of starting cell as mitosis (diploid cells)  Meiosis undergoes the same phases as meiosis (twice)  Meiosis is split into 2 major parts:  Meiosis I: reduction of chromosomes (by ½) by separating homologous pairs  Meiosis II: division of chromatids (just as in mitosis)

The Phases: Meiosis I Prophase I: formation of chromasomes, crossing- over (explained later) Metaphase I: homologous chromosomes line up next to their homologues, spindle fibers begin to attach to centromeres Anaphase I: separation of homologous chromosomes (reduces the # by ½) Telephase I: parent cell begins to divide into 2 daughter cells (cleavage furrow in animal cells, cell plate forms in plant cells) 2 daughter cells are made in Meiosis I 1 Chromosome from each homologous pair can be found in each daughter cell

The Phases: Meiosis II Prophase II: chromosomes remain intact after first division on both new cells Metaphase II: chromosomes now line up single file as they do in mitosis; spindle fibers attach to centromeres Anaphase II: chromatids are divided with one copy of the DNA going each way Telophase II: each daughter cell from the first division now begins to divide The final product of meiosis is 4 daughter cells (all gametes, all haploid) This would make 4 sperm or 1 egg/3 polar bodies (discarded)

Crossing Over  In prophase I of meiosis, homologous chromosomes allow genes to detach and reattach (even on the homologue)  This means that the chromosomes a person receives from his/her parents are not passed in their exact forms on to his/her children, but rather some mixture of the two  Crossing over contributes to genetic diversity

Crossing Over

Mitosis vs. Meiosis  Generates somatic daughter cells  Generates 2 cells  Generates genetically identical copies  Undergoes 1 division  No crossing over  Homologous pairs line up single file  Generates gamete daughter cells  Generates 4 cells  Generates genetically unique copies  Undergoes 2 divisions  Crossing over in prophase I  Homologous pairs line up together in metaphase I